Regex C# is it possible to use a variable in substitution?

Question

I got bunch of strings in text, which looks like something like this:

h1. this is the Header
h3. this one the header too
h111. and this

And I got function, which suppose to process this text depends on what lets say iteration it been called

public void ProcessHeadersInText(string inputText, int atLevel = 1)

so the output should look like one below in case of been called

ProcessHeadersInText(inputText, 2)

Output should be:

<h3>this is the Header<h3>
<h5>this one the header too<h5>
<h9 and this <h9>

(last one looks like this because of if value after h letter is more than 9 it suppose to be 9 in the output)

So, I started to think about using regex.

Here's the example https://regex101.com/r/spb3Af/1/

(As you can see I came up with regex like this (^(h([\d]+)\.+?)(.+?)$) and tried to use substitution on it <h$3>$4</h$3>)

Its almost what I'm looking for but I need to add some logic into work with heading level.

Is it possible to add any work with variables in substitution?

Or I need to find other way? (extract all heading first, replace em considering function variables and value of the header, and only after use regex I wrote?)

You can just use a delegate instead of just a replacement string. — Joey
– Joey, Commented Apr 6, 2017 at 15:12
You can use MatcEvaluator msdn.microsoft.com/en-us/library/… (probably what @Joey is saying) — Imran Saeed
– Imran Saeed, Commented Apr 6, 2017 at 15:16

Wiktor Stribiżew · Accepted Answer · 2017-04-06 16:18:55Z

2

The regex you may use is

^h(\d+)\.+\s*(.+)

If you need to make sure the match does not span across line, you may replace \s with [^\S\r\n]. See the regex demo.

When replacing inside C#, parse Group 1 value to int and increment the value inside a match evaluator inside Regex.Replace method.

Here is the example code that will help you:

using System;
using System.Linq;
using System.Text.RegularExpressions;
using System.IO;
public class Test
{
    // Demo: https://regex101.com/r/M9iGUO/2
    public static readonly Regex reg = new Regex(@"^h(\d+)\.+\s*(.+)", RegexOptions.Compiled | RegexOptions.Multiline); 

    public static void Main()
    {
        var inputText = "h1. Topic 1\r\nblah blah blah, because of bla bla bla\r\nh2. PartA\r\nblah blah blah\r\nh3. Part a\r\nblah blah blah\r\nh2. Part B\r\nblah blah blah\r\nh1. Topic 2\r\nand its cuz blah blah\r\nFIN";
        var res = ProcessHeadersInText(inputText, 2);
        Console.WriteLine(res);
    }
    public static string ProcessHeadersInText(string inputText, int atLevel = 1) 
    {
        return reg.Replace(inputText, m =>
            string.Format("<h{0}>{1}</h{0}>", (int.Parse(m.Groups[1].Value) > 9 ?
                9 : int.Parse(m.Groups[1].Value) + atLevel), m.Groups[2].Value.Trim()));
    }
}

See the C# online demo

Note I am using .Trim() on m.Groups[2].Value as . matches \r. You may use TrimEnd('\r') to get rid of this char.

answered Apr 6, 2017 at 16:18

Wiktor Stribiżew

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1 Comment

DanilGholtsman Over a year ago

Thanks for such full answer, Wiktor!

littlerufe · Accepted Answer · 2017-04-06 15:45:16Z

2

You can use a Regex like the one used below to fix your issues.

Regex.Replace(s, @"^(h\d+)\.(.*)$", @"<$1>$2<$1>", RegexOptions.Multiline)

Let me explain you what I am doing

// This will capture the header number which is followed 
// by a '.' but ignore the . in the capture
(h\d+)\. 

// This will capture the remaining of the string till the end
// of the line (see the multi-line regex option being used)
(.*)$

The parenthesis will capture it into variables that can be used as "$1" for the first capture and "$2" for the second capture

edited Apr 6, 2017 at 15:45

littlerufe

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answered Apr 6, 2017 at 15:28

Vikhram

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1 Comment

Callum Watkins Over a year ago

I don't think this fully answers the OP's question. The h number values need to be manipulated to ensure they are no more than 9 and can be increased by a set amount.

Callum Watkins · Accepted Answer · 2017-04-06 16:35:40Z

1

Try this:

private static string ProcessHeadersInText(string inputText, int atLevel = 1)
{
    // Group 1 = value after 'h'
    // Group 2 = Content of header without leading whitespace
    string pattern = @"^h(\d+)\.\s*(.*?)\r?$";
    return Regex.Replace(inputText, pattern, match => EvaluateHeaderMatch(match, atLevel), RegexOptions.Multiline);
}

private static string EvaluateHeaderMatch(Match m, int atLevel)
{
    int hVal = int.Parse(m.Groups[1].Value) + atLevel;
    if (hVal > 9) { hVal = 9; }
    return $"<h{hVal}>{m.Groups[2].Value}</h{hVal}>";
}

Then just call

ProcessHeadersInText(input, 2);

This uses the Regex.Replace(string, string, MatchEvaluator, RegexOptions) overload with a custom evaluator function.

You could of course streamline this solution into a single function with an inline lambda expression:

public static string ProcessHeadersInText(string inputText, int atLevel = 1)
{
    string pattern = @"^h(\d+)\.\s*(.*?)\r?$";
    return Regex.Replace(inputText, pattern,
        match =>
        {
            int hVal = int.Parse(match.Groups[1].Value) + atLevel;
            if (hVal > 9) { hVal = 9; }
            return $"<h{hVal}>{match.Groups[2].Value}</h{hVal}>";
        },
        RegexOptions.Multiline);
}

edited Apr 6, 2017 at 16:35

answered Apr 6, 2017 at 16:15

Callum Watkins

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1 Comment

DanilGholtsman Over a year ago

Oh wow, interesing, never used it that way before (like in EvaluateHeaderMatch)!

aloisdg · Accepted Answer · 2017-04-06 16:56:44Z

1

A lot of good solution in this thread, but I don't think you really need a Regex solution for your problem. For fun and challenge, here a non regex solution:

Try it online!

using System;
using System.Linq;

public class Program
{
    public static void Main()
    {
        string extractTitle(string x) => x.Substring(x.IndexOf(". ") + 2);
        string extractNumber(string x) => x.Remove(x.IndexOf(". ")).Substring(1);
        string build(string n, string t) => $"<h{n}>{t}</h{n}>";

        var inputs = new [] {
            "h1. this is the Header",
            "h3. this one the header too",
            "h111. and this" };

        foreach (var line in inputs.Select(x => build(extractNumber(x), extractTitle(x))))
        {
            Console.WriteLine(line);
        }
    }
}

I use C#7 nested function and C#6 interpolated string. If you want, I can use more legacy C#. The code should be easy to read, I can add comments if needed.

C#5 version

using System;
using System.Linq;

public class Program
{
    static string extractTitle(string x)
    {
        return x.Substring(x.IndexOf(". ") + 2);
    }

    static string extractNumber(string x)
    {
        return x.Remove(x.IndexOf(". ")).Substring(1);
    }

    static string build(string n, string t)
    {
        return string.Format("<h{0}>{1}</h{0}>", n, t);
    }

    public static void Main()
    {
        var inputs = new []{
            "h1. this is the Header",
            "h3. this one the header too",
            "h111. and this"
        };

        foreach (var line in inputs.Select(x => build(extractNumber(x), extractTitle(x))))
        {
            Console.WriteLine(line);
        }
    }
}

edited Apr 6, 2017 at 16:56

answered Apr 6, 2017 at 16:33

aloisdg

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4 Comments

DanilGholtsman Over a year ago

Latests C# features huh? Still cant force myself to use em

aloisdg Over a year ago

@DanilGholtsman it is just sugar, like lambda instead of delegate.

DanilGholtsman Over a year ago

Yeah, i know, just, you know, hard to get use to it

aloisdg Over a year ago

@DanilGholtsman Added C#5 to compare (we have floating functions and more indentations).

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